Submesoscale mixing on initial dilution of radionuclides released from the Fukushima Daiichi Nuclear Power Plant

福島第一原子力発電所から放出された放射性核種のサブメソスケール渦混合解析

上平 雄基   ; 内山 雄介*; 川村 英之  ; 小林 卓也 ; 古野 朗子   

Kamidaira, Yuki; Uchiyama, Yusuke*; Kawamura, Hideyuki; Kobayashi, Takuya; Furuno, Akiko

本研究では、ダウンスケーリングによる沿岸域の高解像度モデルを導入した高度な海洋拡散予測システムを用いて、福島第一原子力発電所事故に適用し、水平スケールが数キロメートルのサブメソスケールの海象に伴うCsの海洋中移行過程を解析した。その結果、事故直後に福島県沖で発達していたサブメソスケール渦に伴う鉛直循環流によってCsが中深層に活発に輸送されていた。また、これらのサブメソスケール渦はシア不安定と傾圧不安定との相乗的な効果によって強化されていた可能性がスペクトル解析から示された。

We developed a submesoscale eddy-resolving oceanic dispersal modeling system consisting of a double nested oceanic downscaling model and an offline oceanic radionuclides dispersion model to investigate influences of submesoscale coherent structures (SCSs) and associated ageostrophic secondary circulations (ASCs) on the three-dimensional (3D) dispersal and initial dilution of the dissolved radioactive Cs accidentally released from the Fukushima Daiichi Nuclear Power Plant (FNPP1) occurred since March 2011. The extensive model-data comparison demonstrates that the elaborated innermost high-resolution model at a lateral grid resolution of 1 km successfully reproduces transient mesoscale oceanic structures, the Kuroshio path and stratification, and spatiotemporal variations of 3D Cs concentrations. These SCSs and ASCs occurred primarily due to shear instability with baroclinic instability as the secondary mechanism, according to energy conversion and spectral analyses. The vertical Cs flux analysis was performed with decomposition of the variables into the mean, mesoscale, and submesoscale components using frequency and wavenumber filters. The vertical Cs flux analysis explained that 84% of the FNPP1-derived Cs was transported downward below the mixed layer by eddies, with the major contributions from ASCs induced by submesoscale eddies.